The present invention relates generally to the field of magnetic data storage and retrieval systems. In particular, the present invention relates to a thin film transducing head having improved performance due to a reduced thermal pole-tip protrusion and recession.
In a magnetic data storage and retrieval system, a thin film transducing head typically includes a transducer, a substrate upon which the transducer is built, and an overcoat deposited over the transducer. The transducer, which typically includes a writer portion for recording magnetically-encoded information on a magnetic media and a reader portion for retrieving that magnetically-encoded information from the magnetic media, is formed of multiple patterned layers successively stacked upon the substrate. The volume of the transducer is typically much smaller than both the volume of the substrate and the volume of the overcoat.
The layers of the transducer, which include both metallic and insulating layers, all have different mechanical and chemical properties than the substrate. These differences in properties affect several aspects of the transducer. First the layers of the transducing head will be lapped at different rates. Thus, when an air bearing surface (ABS) of the transducing head is lapped during its fabrication, different amounts of the layers will be removed, resulting in the transducing head having an uneven ABS. Commonly, a greater amount of the metallic layers of the transducer will be removed during the lapping process than will be removed from the substrate. Thus, this lapping process results in a pole tip recession (PTR) of the metallic layers of the transducer with respect to the substrate. The PTR of a particular layer is defined as the distance between the air bearing surface of the substrate and the air bearing surface of that layer.
The differing mechanical and chemical properties of the substrate and transducer layers affect the air bearing surface during operation of the transducing head. As the magnetic data storage and retrieval system is operated, the transducing head is subjected to increased temperatures within the magnetic data storage and retrieval system. In addition, a temperature of the transducing head itself, or a part thereof, may be significantly higher than the temperature within the magnetic data storage and retrieval system due to heat dissipation caused by electrical currents in the transducer.
The coefficient of thermal expansion (CTE) of materials used in forming the substrate is typically much smaller than the CTE of materials used in forming the metallic layers of the transducer. Due to the large CTE of the transducer's metallic layers, those layers tend to expand a greater amount in response to high temperatures than the substrate. Thus, when the transducing head is subjected to high operating temperatures, the metallic layers tend to protrude closer to the magnetic disc than the substrate, thereby affecting the PTR of the transducer. This change in PTR caused by temperature is referred to as the Thermal PTR (TPTR).
During operation of the magnetic data storage and retrieval system, the transducing head is positioned in close proximity to the magnetic media. The distance between the transducer and the media is preferably small enough to allow for writing to and reading from the magnetic media with a large areal density, and great enough to prevent contact between the magnetic media and the transducing head. Performance of the transducer depends primarily upon the distance between the media and the transducing head.
To keep the distance between the transducing head and the magnetic media constant, PTR should not change significantly with temperature. If TPTR is large, then the spacing between the transducer and the media will change significantly with temperature, thereby requiring that the low temperature fly height be enough to accommodate the higher operating temperatures. On the other hand, if TPTR is close to zero, the low temperature fly height can be reduced.
TPTR has become an increasingly significant problem as head-media spacing (HMS) continuously decreases for higher density magnetic recording. The specified operating temperature range for drives is excess of 50° C. and the transducer temperature varies linearly with and is higher than the ambient air temperature inside the drive by 10° C. or more. Long write operations also raise head temperatures significantly by joule heating of coils. Current transducing heads contain many materials with a higher CTE than the substrate, commonly made of AlTiC. The materials of the transducing heads include permalloy, CoNiFe, gold, copper, and photo resist. Relative to the AlTiC substrate, these high CTE materials cause transducing head structures to protrude when the temperature rises and recess when the temperature drops. A common definition of TPTR is protrusion/recession distance per unit temperature change, generally in n″/° C.
TPTR affects magnetic transducer performance primarily in two ways. First, pole-tip protrusion at elevated temperatures increases the possibility of head disc contact. TPTR has proven to be a significant contributor to trailing edge wear of a slider. Mechanical reliability risk caused by large TPTR prohibits a low fly height and reduction in pole-tip recession. Low TPTR is one of the enablers for continuous HMS reduction, critical to higher recording areal density. Second, TPTR contributes to the cold write problem. Pole-tip recession at cold temperatures increases HMS. This degrades writablility, signal-to-noise ratio, and bit error rate at cold temperatures.